Accept upper-case base32 in the pure-Python decoder

[gaoflow]

The library ships the same public API twice: the _geohash Rust/PyO3 extension and the pure-Python geohash.py fallback used when the extension is not importable. The extension's build_base32_decode_map (src/geohash_core.rs) deliberately accepts upper-case base32 letters, but the fallback's _base32_map is lower-case only, so the decode path behaves differently depending on whether the compiled extension is present:

>>> import geohash                      # extension not built
>>> geohash.decode('EZS42')
Traceback (most recent call last):
  ...
KeyError: 'E'
>>> geohash.decode('ezs42')
(42.60498046875, -5.60302734375)

Every entry point that decodes a code -- decode, decode_exactly, bbox, neighbors, expand -- goes through _decode_c2i, so all five raise a bare KeyError on an upper-case geohash in the fallback while the extension returns a result. This is a widely hit path: PyPI ships a single x86_64 Linux wheel plus the sdist, and the README documents copying geohash.py standalone, so macOS / Windows / aarch64 / musllinux installs and the standalone-copy use all run the fallback.

This populates _base32_map with the upper-case keys as well, mirroring the extension's map exactly -- only the base32 letters get an upper-case form, so a i l o and A I L O stay invalid and mixed case works per character. It also replaces the bare KeyError for an out-of-alphabet character with the same ValueError("geohash code is [0123456789bcdefghjkmnpqrstuvwxyz]+") the extension raises through map_error (src/lib.rs).

Tests add upper-case, mixed-case and invalid-character cases across all five decode entry points. I also cross-checked the fallback against the built extension over 12k lower/upper/mixed geohashes and 3k invalid inputs; value and error type now match on every case, and the existing tests still pass.